1. Field of the Invention
This invention relates to the field of battery charging, and particularly to methods and systems for battery charging using a specified charge profile.
2. Description of the Related Art
Rechargeable lead-acid batteries are charged while their DC loads are disconnected, or while their loads are connected. The former would be represented by lift-truck and golf cart batteries, while the latter would include engine-powered road or rail vehicles. Battery chargers provide a current to and a voltage across the battery in accordance with a predetermined ‘charge profile’. Freight and passenger locomotives typically use a 32 cell battery; a generator powered by the traction engine is used to recharge the battery after the engine is started and to keep the battery charged while simultaneously supplying the various DC auxiliary loads (lights, air conditioning, heating, computer, etc.) while the locomotive is in service.
EMD (Electro-Motive Diesel Division of Caterpillar Corp, formerly Electro-Motive Division of General Motors Corp.) locomotives manufactured prior to 2004, typified by Models SD40 through SD70, have a 74 V electrical system that is powered by a dedicated voltage-regulated “auxiliary generator”. This may be a DC machine, or an AC machine with a diode rectifier. The charge profile is one-stage, with the DC generator or alternator/diode rectifier voltage regulated to a nominal 74 V by controlling the generator field current. Charge current is limited only by the battery and cable resistance, plus an added 50 mohm resistance. On EMD locomotives manufactured after 2004, such as the SD70ACe, the 74 V electrical systems are powered from a thyristor rectifier that is sourced with variable frequency/variable voltage AC power from a “companion alternator” that also provides AC power to drive the locomotive cooling fans and blowers. The charge profile provided by the thyristor rectifier is one-stage, with voltage regulated to a sometimes temperature-compensated 74 V. The total rectifier current is electronically limited to 425 A, but the battery current is limited by cable resistance and an added 50 mohm resistance.
A prototype locomotive made by the Progress Rail Division of Caterpillar was recently placed in service with a three-stage charger profile. In this charger, the first stage (referred to as the ‘Bulk’ stage) current is limited to 80 A, the second ('Absorption') stage voltage is limited to 78 V, and the third (‘Float’) stage current is limited to 5 A.
Conventionally, the transition between the Absorption and Float stages of a three-stage charger profile is made when the charging current has decreased to a fraction of the initial current. The transition threshold for the current must be high enough to ensure that the Absorption Stage charge current drops below the threshold under the worst-case combination of high temperature and degraded battery condition; if the transition threshold is too low or if the battery is so warm that the charge current never reaches the threshold, a condition known as ‘thermal runaway’ can occur which can destroy the battery. Because of the necessarily high transition current threshold, the Absorption Stage of conventional three-stage charge profiles will terminate before the all of the active material in the battery has been converted. If not all the material is converted (lead→lead sulfide), a sulfur compound will build up on the battery's plates, which degrades the battery's ability to accept charge.
One pending patent application (US 2009/0218990 to Johnson et al.), describes how the rate-of-change of current can be used to determine the time of the Absorption/Float Stage transition. This approach uses the difference of sampled current values divided by the time between samples to determine the di/dt of the charge current. This form of Absorption/Float Stage transition is used in golf cart and similar applications where the battery does not supply a load while being charged. However, using di/dt as an Absorption/Float Stage transition criterion can be problematic in the locomotive application, where live DC loads are switched on and off while the locomotive is in service. Owing to the finite response time of the voltage and current regulator circuits (typically 1.0 s for Auxiliary Alternator powered systems), the charging current and voltage are constant only when the load current is constant. When a high current load (for example, the cab heater) is switched on, the battery current momentarily changes from positive (charging) to negative (discharging). Switching the high current load off has the opposite result; battery current steps to a more positive value and then momentarily rebounds to a negative current. Since battery current is highly disturbed for a time after the load transient, the usefulness of using di/dt as a preventer of thermal runaway may be limited.
Several patents that are not directly related to 3-stage charging propose using the rate-of-change of battery voltage to control the charge profile; examples include U.S. Pat. No. 4,3923,101 to Saar et al. and U.S. Pat. No. 7,589,491 to Brecht. An article entitled “Charge Batteries Safely in 15 Minutes by Detecting Voltage Inflection Points”, Cummings et al., EDN, Sep. 1, 1994, describes how the battery voltage dv/dt can be used to control charging. However, it is doubtful that using battery voltage dv/dt to control the charge profile would be possible if, as in the locomotive application, loads are connected and disconnected while the battery is being charged.
Other patents, such as U.S. Pat. Nos. 5,214,370 and 6,020,721, describe a means of reducing the possibility of thermal runaway through the use of sensors that adjust the battery voltage as a function of ambient or battery temperature. However, locomotive operators have found it problematic to measure the actual battery temperature with a delicate temperature probe in the rugged locomotive environment.
Use of a one-stage charge profile as described above can adversely affect the performance characteristics of a locomotive battery, such as the ability to support lighting and other loads when parked with the engine not running, or the ability to provide adequate (1000+ A) engine cranking current, especially at low ambient temperature. Locomotive operators report that batteries on diesel-electric locomotives that are shut down and restarted multiple times per day due to fuel costs or pollution regulations experience high cell failure rate, high water consumption and premature loss of capacity. Locomotives equipped with automatic engine start systems (AESS) are especially prone to battery problems. The short battery life associated with frequent engine starting can be traced to excessive current demand that occurs immediately after the engine starts, and insufficient voltage after the battery charger voltage reaches the typical 74 V setpoint. Excessive initial current creates localized thermal stress on the battery plates, and insufficient voltage results in incomplete conversion of the battery's active material during the time that the engine is running.
The charge current and battery voltage for a typical one-stage charge profile is illustrated in FIG. 1. Only the rectifier voltage (1) is regulated in the one-stage charger. Sufficient resistance is present in the battery circuit to limit the initial charging current to a level that allows the generator to produce enough power for the rectifier to reach its typical 74 V setpoint immediately after the engine starts. The initial battery current (2) is determined by the difference between the rectifier voltage and the battery's internal charge-dependent EMF divided by the DC circuit resistance. The latter consists of about 20 mohm of cable and battery internal resistance plus, in the case of EMD locomotives, an added 50 mohm resistor, for a total DC resistance of about 70 mohm. If the battery is highly discharged when the engine starts, a damaging current approaching 150 A can flow for a time after the engine starts. The 70 mohm of DC resistance creates a substantial voltage drop even as current decreases with charging time; for example, after an hour with the rectifier at 74 V, current might have decreased to 50 A. With 70 mohm of DC resistance, the battery voltage (3) will have risen to only 74−(50*0.07)=70.5 V. This is well below the manufacturers recommended value of 74 V at 25° C. (GNB Industrial Power Application Note, “KDZ-501 Charging Strategy”). As current decreases further, battery voltage increases and the current stabilizes at temperature dependent levels of (4) or (5) for low and high ambient temperatures, respectively. The risk of thermal runaway at high ambient temperature can be seen in the upward trending curve of current vs. time for the warm battery. Low ambient temperature causes the downward-trending current curve; a trend that indicates a loss of capacity. Battery manufacturers recommend using a battery temperature probe and associated regulator circuitry to decrease the charge voltage as a function of temperature. However, the delicate construction of the temperature probe and wiring cause locomotive operators to generally ignore the battery manufacturers' recommendations.